Re: [libvirt] [PATCH V2] qemu: Redefine the "unlimited" memory limits one more time

On 25.11.2016 17:14, Daniel P. Berrange wrote: > On Fri, Nov 25, 2016 at 05:12:41PM +0100, Martin Kletzander wrote: >> On Fri, Nov 25, 2016 at 03:39:17PM +0000, Daniel P. Berrange wrote: >>> On Fri, Nov 25, 2016 at 04:35:14PM +0100, Martin Kletzander wrote: >>>> On Fri, Nov 25, 2016 at 03:19:54PM +0100, Viktor Mihajlovski wrote: >>>>> On 18.11.2016 17:44, Viktor Mihajlovski wrote: >>>>>> With kernel 3.18 (since commit 3e32cb2e0a12b6915056ff04601cf1bb9b44f967) the >>>>>> "unlimited" value for cgroup memory limits has changed once again as its byte >>>>>> value is now computed from a page counter. >>>>>> The new "unlimited" value reported by the cgroup fs is therefore 2**51-1 pages >>>>>> which is (VIR_DOMAIN_MEMORY_PARAM_UNLIMITED - 3072). This results e.g. in virsh >>>>>> memtune displaying 9007199254740988 instead of unlimited for the limits. >>>>>> >>>> >>>> Ah, I wonder hoe many times we'll have to deal with this. >>>> >>>> Anyway, this means it is not enough to shift by 2 if the system hugepage >>>> is more than 4k (e.g. 2M). I remember we had to change something due to >>>> such hosts. virGetSystemPageSize(KB) should help with that. >>>> >>>> We could also make it 2^50 - pagesize just to make sure it will work for >>>> a while, but some might not like it. >>> >>> I guess I'm not understanding how this code copes with the fact that >>> we now have 3 different "unlimited" values to deal with. >>> >>> Could we not simply record the unlimited values and pick the right >>> one based on kernel version we detect ? >>> >> >> We have one, which we return for Get APIs that means that the setting >> was not restricted. We would need to have a cgroup which we set to >> some huge value and then read it, store it and compare it all the time. >> It _should_ work as far as I can tell. > > No, I meant can we use some constants > > #define UNLIMTED_VALUE_KERNEL_A_B_C 2343243245325 > #define UNLIMTED_VALUE_KERNEL_D_E_F 3253253253253253 > #define UNLIMTED_VALUE_KERNEL_G_H_I 325325325325232 > > > if kver == A_B_C && value == UNLIMTED_VALUE_KERNEL_A_B_C > value = VIR_DOMAIN_MEMORY_PARAM_UNLIMITED > else if kver == D_E_F && value == UNLIMTED_VALUE_KERNEL_D_E_F > value = VIR_DOMAIN_MEMORY_PARAM_UNLIMITED > else if kver == G_H_I && value == UNLIMTED_VALUE_KERNEL_G_H_I > value = VIR_DOMAIN_MEMORY_PARAM_UNLIMITED > > > Regards, > Daniel > Probably easier to read, but relying on the reported kernel version alone is not safe as it is not uncommon to find backports introducing functionality from later kernels.

We could instead try to probe the max value. IIUC, the root memory limit cannot be modified from userspace, so this might be the right value to look up. I can craft something in this direction, if you agree with the approach.

-- Mit freundlichen Grüßen/Kind Regards Viktor Mihajlovski IBM Deutschland Research & Development GmbH Vorsitzender des Aufsichtsrats: Martina Köderitz Geschäftsführung: Dirk Wittkopp Sitz der Gesellschaft: Böblingen Registergericht: Amtsgericht Stuttgart, HRB 243294